This invention relates generally to a heat treat process and more particularly to a carburizing heat treat process using ions in a gaseous atmosphere to bombard the surface of a ferrous workpiece to achieve a carburized case surface.
The invention is thus particularly applicable to carburizing by means of a glow discharge technique in a vacuum and will be discussed with particular reference thereto. However, the invention may have broader application in that it may be utilized in any ion glow discharge treatment process where the gaseous atmosphere has high electrically conductive characteristics such as that which may be encountered in boronizing and certain metal plating processes.
Carburizing the case of a ferrous workpiece has traditionally been accomplished by either atmosphere or vacuum heat treat furnaces. Generally, atmosphere furnaces can perform a wide variety of heat treat processes but cannot achieve the dimensional tolerance control that vacuum furnaces provide in carburizing ferrous workpieces. When carburizing is performed in either a vacuum or atmosphere furnace, a carrier or inert gas is mixed with a carbon bearing gas, such as methane or propane, which disassociates at high temperatures to diffuse carbon into the case of the workpiece to give the surface a hard, toughened wear characteristic. The presence of a carrier gas, in and of itself increases the cost of the process and tends to increase the overall processing time to a greater value than that which might otherwise be possible.
For a number of years, principles related to the ionization of gases have been used in a vacuum chamber with a DC current established between the workpiece cathode and the vacuum chamber anode to cause an ionic bombardment of the workpiece's surface by a disassociated ammonia gas to produce an iron nitride case on the workpiece. Use of such "glow discharge technique" has proven commercially superior to vacuum and atmosphere furnaces when performing nitriding heat treating processes. Demonstrated advantages include closer dimensional control of the workpiece and the fact that irregular surfaces on the workpiece, such as blind holes, can be uniformly treated with a nitride case. Similar advantages, albeit perhaps not as significant, are expected and have been realized in an ion carburizing process.
Recently, a number of attempts have been made to commercially use the "glow discharge technique" for carburizing ferrous workpieces with varying degrees of success. The problem uniformly encountered on a commercial basis can be defined as consistency. That is, almost any given geometrical configuration of a single workpiece can be carburized utilizing conventional glow discharge apparatus and processes. However, when a wide variety of parts must be treated over the life of the furnace and the parts are simply placed in a basket within the furnace it has not been possible to consistently carburize the parts despite the successful history of the glow discharge technique in nitriding and despite the numerous publications covering the glow discharge process, most of which simply treat the nitriding and carburizing processes as identical for glow discharge purposes. There are several problems which have been encountered. One significant problem encountered in ion carburizing (as well as in all ionizing processes) is that of "fireballs". A fireball occurs when the glow discharge seam runs amok and results in a ball of fire positioned over some discrete area of the workpiece. More specifically, a fireball is attributed to a localized arc which does not short circuit the system. Thus normal electrical controls which would otherwise sense arcing about the entire workpiece to produce a short circuit are ineffective to control the fireball phenomenon. Other significant problems encountered in ion carburizing relate to the inability to achieve a consistently uniform carbon case and the inability to achieve reasonably fast processing times.
The problem of arcing or fireballs is particularly acute in carburizing, because the carburizing gas upon disassociation produces an atmosphere which is electrically conductive while the atmosphere produced in nitriding from disassociated ammonia is electrically non-conductive. To minimize arcing tendencies associated with the use of such electrically conductive atmosphere, current attempts to ion carburize ferrous workpieces dilute the carbon bearing gas (methane, propane, etc.) with an inert, electrically nonconductive carrier gas (hydrogen, nitrogen, etc.). While the arcing tendency attributed to the atmosphere is thus reduced, the time for the process to achieve carburization is increased because both carbon bearing and non-carbon bearing gases must be ionized.
All glow discharge furnaces utilize some mechanism for controlling the current to avoid localized arcing which produces fireballs. In one commercially successful nitriding glow discharge process, the current is interrupted whenever (i) the current exceeds a predetermined value, or (ii) whenever the voltage change over a time change exceeds a certain predetermined value, or (iii) whenever the voltage change with respect to the current change over a timed increment exceeds a predetermined value Another approach, such as disclosed in U.S. Pat. No. 4,490,190 to Speri, uses a pulsed current, produced by an interruptor circuit from either direct current or rectified single or multiphase alternating current, without any additional arc control to produce the glow discharge. In the pulsed current approach of Speri, which uses an alternate source of heat such as disclosed in U.S. Pat. No. 4,124,199 to Jones to heat the workpiece, the pulsed current per se, is viewed as sufficient to prevent arcing or fireballing of the workpiece and the wattage of the power source is simply increased until the glow discharge is produced. Still another approach, disclosed in U.S. Pat. No. 4,331,856 to D'Antonio, used in a nitriding process to control arcing is to use a comparator circuit to measure the workpiece temperature and change thereof and when the change or temperature limits are exceeded the glow current is stopped. Another approach at controlling the current is disclosed in U.S. Pat. No. 4,587,458 to Davenport et al where a third dummy electrode is used to control the current actually imparted to the workpiece.
In using a vacuum, glow discharge heat treating furnace for commercially carburizing workpieces in a basket, whether of the same or dissimilar configuration in a batch mode, none of the existing processes including those described above has proven satisfactory. The processing time for carburizing was excessive when compared to the processing time of conventional vacuum furnaces, or the fireballing or arcing phenomenon prevented the process from going forward or required the power to the vessel to be reduced to a lower level than that which is otherwise available to reduce arcing or fireballs so that the time for processing is extended so as to be unsatisfactory from a commercial viewpoint or the carburized case depth was not uniform. This conclusion was reached after the traditional parameters related to controlling ion processes (as cited) along with traditional parameters such as flow rate, pressures and temperatures which are considered in vacuum and atmosphere carburizing processes were varied, mixed and matched in an attempt to produce a successful commercial ion carburizing process.